Radial velocities in W5 YSO groups

There are three dense groups in the cavities of H ii bubbles, and the other five sparse groups are distributed at the edges of the bubbles. The three dense groups have almost the same age (5Myr) and show a pattern of expansion. The scale of their expansion is not large enough to account for the overall structure of W5.

The three northern groups are, in fact, 3Myr younger than the dense groups, which indicates independent star formation events. Only one of these groups shows the signature of feedback- driven star formation as its members move away from the eastern dense group. The other two groups might have formed in a spontaneous way.

On the other hand, the properties of two southern groups are not understood as those of a coeval population. Their origins can be explained by dynamical ejection of stars and multiple star formation. Our results suggest that the substructures in W5 formed through multiple star-forming events in a giant molecular cloud.